images in cross-platform stacks

[Thomas McGrath III]

Concerning PNGs and Gamma,

This behavior of PNGs also occurs when using PNGs in a PocketPC  
device. In recent development I had PNGs with transparency "self"  
adjusting by as much as 15 points. I started out with pure white  
255,255,255 for transparency and an off white 250,250,250 for non  
transparent white and in the PocketPC device the 250,250,250 was  
turning transparent. I had to adjust the off white down to  
240,240,240 before it would no longer turn transparent. This was not  
the case on the MAC or PC.

The gamma on PocketPC is different than on Windows which is different  
than on Mac. The other thing too is that the PocketPC display screen  
is not 72-dpi but actually much higher. The device I am using is 192- 
dpi. quote MSDN "Windows Mobile™-based devices have traditionally  
used 96-dpi displays, but Windows Mobile 2003 Second Edition-based  
devices can run at higher resolutions."

------ It seems the otherwise-industry-standard is not the same  
across the industry for gamma or screen definition.

GAMMA problem:    http://hsivonen.iki.fi/png-gamma/
"In order to compensate for the different gammas, the display gamma  
of the system an image was authored on and the display gamma of the  
system where the image is being displayed both need to be known to  
the piece of software displaying the image. The PNG format provides a  
means for storing the (reciprocal of) display gamma of the authoring  
system to the image file. It is up to the program writing the file to  
store the native display gamma of the image, and then it is up to  
program displaying the file to compensate.
The new problem is that the programs that read and write PNG files  
usually have no idea about the display gamma of the system they are  
running on. The usual method is to take a guess based on the platform  
the program is running on and pick one of the default values  
mentioned for Mac, SGI, NeXT and PC clones in the Gamma Tutorial  
appendix of the PNG specification.  http://www.w3.org/TR/PNG/

This approach is bogus. The platform defaults are only that:  
defaults. The display gamma is configurable on many systems. Also, on  
uncalibrated systems the default values are not exact. Writing bogus  
gamma information to files and then doing gamma “correction” using  
different bogus gamma information is not solving the problem—only  
perturbing it further.

In practice, a rather arbitrary gamma change is often applied to a  
PNG image when the image is opened in a program that did not write  
the file. When a PNG image is viewed in isolation, the gamma  
disturbance may not be obvious. However, when the colors of an image  
are meant to match some surrounding colors—such as the colors of a  
Web page—the problem becomes more serious."


GAMMA is different across platforms, devices, and users screens and  
is treated differently by the application that created the PNG and  
the application that reads the PNG. We are actually fortunate that we  
can set this for use in REV based on the w3 guidelines BUT as you can  
see it is only a best guess.


HTH

Tom



On May 17, 2006, at 2:05 AM, Richard Gaskin wrote:

>
> Tereza Snyder wrote:
>
>> On May 16, 2006, at 10:47 PM, Richard Gaskin wrote:
>>> Rev doesn't alter the display of JPEGs or GIFs -- why single out   
>>> PNGs in this way, and why try to adjust for screen gamma when  
>>> the  screen is already doing anything that needs to be done there?
>> and Mark Talluto wrote:
>>> This is a question that goes way back to the good Dr. Raney.  I   
>>> remember having a discussion with him about it.  The details are   
>>> fuzzy, but they went something like this:  That is the way it is.
>> I seem to remember something too in the dim past. PNGs, I know,   
>> contain gamma info in their headers in contrast to JPEGs, which  
>> may  not. The info is used by rev to display consistent gamma  
>> cross-platform.
>> In the present, I take advantage of it because it shows, on the  
>> Mac,  the gamma I can expect in Windows. My applications are wall- 
>> to-wall  images. I use FireWorks to make most of my images and set  
>> it to  display Windows gamma. I achieve better control over  
>> contrast because  I see what (most of) the end users will get.
>> It's on purpose!
>
> Fireworks, eh?  One more thing I like about you. :)
>
> It never occured to me that PNGs would try to compensate for  
> differences  in gamma.  I guess I'm just one of those old fashioned  
> guys who thinks that if a person wants a different gamma they can  
> just change it for everything, rather than have some elements not  
> match others.
>
> Why doesn't Mac use the otherwise-industry-standard gamma?
>
> Or perhaps more appropriately now that we're in a world where Apple  
> ships with two-button functionality and runs on Intel, WHEN will  
> Apple change their default gamma?
>
> I went out on a limb several years ago and predicted the shift to  
> Intel and two-button mice for Apple; I'll put myself out there  
> again and suggest that Apple may adopt the rest of the world's  
> default gamma within 24 months.
>
> -- 
>  Richard Gaskin
>  Fourth World Media Corporation

Thomas J McGrath III
3mcgrath at adelphia.net

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